1. Field of the Invention
The invention relates to a joined structure of a ceramic member and a metal member.
2. Related Art Statement
Electrostatic chucks have been used to adsorb and hold semiconductor wafers in transferring, light-exposing, film-forming such as CVD and sputtering, fine processing, washing, etching, dicing, etc. for semiconductor wafers. Dense ceramics are noted as substrates for such electrostatic chucks. Particularly, in a semiconductor producing apparatus, a halogen based corrosive gas such as ClF3 is often used as an etching gas or a cleaning gas. In order to rapidly heat and cool a semiconductor wafer while being held by such a substrate, the substrate of the electrostatic chuck desirably has a high heat conductivity. Further, it is desired that the substrate has such thermal shock resistance as not to be destroyed by rapid temperature changes. Dense aluminum nitride and alumina have high corrosion resistance against the halogen based corrosion gas mentioned above.
In the field of the semiconductor producing apparatuses, susceptors in which built-in high frequency electrodes for generating plasma have been practically used. In the field of such high frequency electric power generating apparatuses, a metal electrode is embedded in a substrate of aluminum nitride or dense alumina. Further, in the field of the semi-conductor producing apparatuses, there are used ceramic heaters in which a metal resistor is embedded in a substrate of aluminum nitride or alumina for controlling the temperature of a wafer in each process.
In such devices, it is necessary to embed a metal electrode in a substrate of a ceramic material such as aluminum nitride or the like and to electrically connect the metal electrode to an outer connector for supplying electric power. The connecting portion of the metal electrode and outer connector is exposed against thermal cycles of high and low temperatures under an oxidizing atmosphere or corrosive gas atmosphere. It is desired to preserve good electrical connection and a high joining strength over a long time under such severe circumstances.
The applicant filed a Japanese patent publication No. 10-209, 255A, and disclosed a connecting structure of a connector and a metal electrode. In the structure, a conductor of a low thermal expansion coefficient is inserted between the metal and ceramic members to relax the residual stress remained in the connecting portion. The applicant has further disclosed an improved structure based on that of the Japanese patent publication No. 10-209, 255A, in Japanese patent publication No. 11-12, 053A. In the structure, a soft brazing material of an alloy containing gold, platinum, or palladium is used to improve the anti-oxidation property of the brazing material and to further relax the residual stress.
The following problems, however, may occur when a heater or an electrostatic chuck having the above described joining portion is manufactured in a large scale. That is, when the joined structure is produced, a metal brazing material such as a solder of the alloy of gold is provided between the conductor of a low thermal expansion coefficient and the surface of the ceramic member. The brazing material is then heated and melted to wet the surface of the ceramic member and that of the conductor of a low thermal expansion coefficient, so that they are joined with each other. The thickness of the resulting joining layer tends to be, however, variously changed. The reasons are as follows. It is relatively difficult to sufficiently wet the surface of the ceramic member so that the molten solder tends to flow out of the joining layer. Since the molten brazing material is leaked out of the joining portion, it is difficult to accurately control the amount of the molten brazing agent remaining in the joining portion. Consequently, the thickness of the joining layer is fractuated or sometimes reduced to a thickness of 0.05 mm or lower. When the thickness of the joining layer is small, thermal cycles applied on the joining layer may induce a substantial stress along the interface of the joining layer and ceramic member. Cracks may be generated in the ceramic member or joining layer so that the resulting joined article is out of a specification.